The field of the invention pertains to the interference fit assembly of metal products and, in particular, to the interference fit assembly of iron and steel parts such as collars and bearings on shafts although the invention disclosed below is not limited to collars and bearings on shafts.
Iron and steel exposed to an oxidizing environment form an oxide surface layer. Over time the oxiding surface continues to thicken eventually causing the complete oxidation of the metal because the initial oxide layer does not protect the substrate metal from further oxidation. The cross-diffusion of iron and oxygen through the oxide layer permits the substrate metal to oxidize beneath the oxide layer in the vast majority of applications the oxidation of iron and steel is considered very detrimental because of the discoloration, degradation and eventual destruction of the iron or steel product. A purposely formed oxide layer has been disclosed in U.S. Pat. No. Re. 21,903 wherein the oxide layer frictionally resists the untwisting of the inter-twisted ends of wire strapping.
At relatively low temperatures the oxide layer appears to be predominately Haematite (Fe.sub.2 O.sub.3) and Magnetite (Fe.sub.3 O.sub.4). However, at relatively high temperatures above 570.degree. C. (1058.degree. F.) according to N. Birks, "Introduction to High Temperature Oxidation of Metals", Edward Arnold Press, London, 1983, pp. 74-75, the oxide layer becomes overwhelmingly Wustite (FeO) with only very thin Haematite and Magnetite films. According to this reference Wustite does not form below 570.degree. C. but reaches 95% of the oxide layer thickness at 1000.degree. C. (1832.degree. F.). Between 400.degree. C. (752.degree. F.) and 570.degree. C. the oxide layer is apparently formed of .alpha. Fe.sub.2 O.sub.3 (rhombohedral structure) and .gamma. Fe.sub.2 O.sub.3 (cubic structure) because the Fe.sub.3 O.sub.4 oxidizes to form .alpha. Fe.sub.2 O.sub.3. The differing colorations and characteristics of scales formed on iron and steel at various temperatures are a result of the complex chemical and crystal structures formed in the oxide layer.
Numerous mechanical means of retaining collars on shafts have been disclosed historically, such as the means disclosed in U.S. Pat. Nos. 3,118,711; 3,328,096; 3,428,373; 3,535,008; 3,697,145; 4,090,746 and 4,166,661. Such mechanical means all require additional operations and parts which add to manufacturing cost. However, the simplest means of retaining a collar on a shaft remains an interference fit and interference fits remain common means of assembling a variety of mechanically fastened components.
Interference fits rely upon a combination of frictional engagement and squeezing tightness to prevent disassembly in theory the squeezing tightness causes a more intimate contact of the microscopically rough surfaces thereby substantially increasing the force required to overcome the frictional forces opposed to disassembly of the interference fit. Moreover, the more intimate contact of the microscopically rough surfaces increases the number of sites of microscopic welding between surfaces thereby further enhancing the frictional resistance to disassembly.